Seems to me its either web server problem or routing problem - can you give me your routing rules.. Or there might be a session / cookie problem. Try with different browser, or try cleaning your cache.

Unable to login to the Gii interface. Pretty vanilla setup. Followed this and every other thread that tries to help with this issue on this site. Have edited the PHP.INI file as suggested above for Windows. It seems to me that someone should have a simple solution, which would allow for Gii to work simply by uncommenting the module in the main.php configuration file. Instead, literally hours of precious life is lost trying to get working, what should be a minor step in using yii on a development server.

I had a same problem finally got it fixed. This was PHP's session problem, as you mentioned, in the php session directory (in my case, /var/lib/php/session) the web server user (daemon) did not have the permission to write in.

I chowned the directory from root to daemon and it worked fine.
(Fedora)